Accurate modeling and design of LLC resonant converter with planar transformers

In order to design and fabricate low profile (slim) LLC resonant converters, it is essential to replace conventional wire-wound magnetic transformers with low profile planar ones. In planar transformers, to decrease high-frequency AC resistance and to get high power efficiency, the primary and secondary PCBs are stacked together using the interleave structure method, which leads to a high amount of inter-winding and intra-winding capacitances. These winding capacitances can change the behavior of the LLC resonant converter and disturb the soft transition of the power MOSFETs, because the resonant circuit current should charge and discharge not only the effective Drain-Source capacitances of the power MOSFETs, but also the planar transformer winding capacitance. In this paper, the steady state equations of the LLC resonant converter with the planar transformer are extracted. In order to fully consider the characteristics of the LLC resonant converter, a new set of analytical equations is obtained for the LLC resonant converter, with consideration of the planar transformer equivalent capacitance. The experimental results extracted from a 36VDC, 200W prototype platform verify the accuracy of the analytical equations obtained, and demonstrate the validity of the employed model.